Battery banks are storage devices that store potential electrical energy in a chemical form. Battery banks are available as two types: rechargeable and non-rechargeable. Traditional rechargeable chemistries are lead acid, NiCd, Nimh, NiH2, and AgO—Zn. New promising battery chemistries are Li—ION, Li-POLY, Li—FePO4, as well as EDLC super-capacitors. The storage banks are typically created by connecting multiple low voltage storage cells in series to produce the required terminal voltage. Slight differences in manufacturing and temperature of each of the storage cells can over time lead to cell voltage mismatches in the stack. The traditional battery cell chemistries by nature are self balancing: modest overcharging of one cell is dissipated by heat allowing the others in the series stack to catch-up. The new lithium chemistries and super-capacitors do not have the natural self-balancing functionality. If allowed to overcharge, the cells can catastrophically fail. Balancing eliminates this weakness by forcing all of the cells to charge to the same cease charging threshold. Balancing also maximizes the potential energy to be recovered during discharge thus increasing the total storage efficiency. This leads to longer per cycle use as well as increased total battery life. Even traditional battery chemistries can benefit from active balancing over the traditional overcharge method.
Cell balancing has traditionally been accomplished with the use of resistive shunts to balance cell voltages evenly. This is accomplished by parasitically draining energy from the cells with the higher state of charge (SOC) to drop them to the level of the cells with a lower SOC. This is rather inefficient as energy in cells with higher a SOC is shunted as heat while energy is continued to be added to the bank as a whole to continue and charge the lower SOC cells. This is known as the dissipative method, and is current practice in the aerospace market as well as small terrestrial battery storage systems.
In one such dissipative balancer the electronics performs cell balancing on each of the cells by means of resistive bypass around the cell. When any cell voltage reaches a predetermined threshold a resistor is placed in parallel with the cell, bypassing about 100 mA. The bypassing terminates when the cell voltage drops below a second lower predetermined voltage. There are a non-trivial amount of components required to accomplish this balancing procedure. This impacts the system twofold, one, the total system reliability, and two, the reduced system efficiency by dissipating power while charging. In a situation with a severely unbalanced bank, charging must periodically cease to allow the stronger cells to drop to the level of the weakest before charging can resume. This represents lost time and energy to be stored. With high quality aerospace batteries the balancing disparities are normally tiny due to cell lot testing and manufacturing needing little to no active balancing. In the commercial world, battery banks are assembled from multiple lots and or venders; thus continuous balancing is necessary.
The commercial world has used two approaches to improve the efficiency of cell balancing. The first is transferring energy from the entire bank as a whole to the cells with lower SOC. This method requires individual switching converters or a ganged fly-back converter to transfer this energy. There is the disadvantage that a dissipative balancer is still needed to address an overcharged cell. The dissipative balancer can be replaced with additional unidirectional converters to remove excess charge and return it to the whole bank.
The second more complex approach actively shuttles energy from the high SOC cells to the lower ones. This is usually accomplished by either transferring energy through the adjacent cells or through an auxiliary transfer bus. This requires some sort of digital control scheme to redistribute the energy when there are multiple steps to move the energy. The energy shuttling is in the form of switched capacitor or inductive charge storage.
These methods require direct galvanic connection of the balancer to each cell in the battery bank. Part count and reduced system reliability become a factor with the more complex balancing methodologies. The active energy shuttling methods represent the most efficient manner for redistributing the energy already contained within the battery bank from the external charger. One producer of high reliability space oriented electronics has such a common share bus balancer but the device is heavy and power hungry.